The present invention relates to a process of preparing a metal supported catalyst by supporting various catalyst metals on a support, especially to a process of reducing an ion containing the metal.
A catalyst comprising an inorganic oxide support such as silica and alumina, and an individual noble metal such as platinum, gold and palladium or the combination thereof supported thereon has heretofore been employed as that for various chemical reactions and for an electrode of a fuel cell. Another catalyst comprising a carbon support and the same catalyst noble metals supported thereon has been also employed.
The catalyst performance of these catalysts depends on the degree of dispersion of the catalysts metals and the performance (specific activity) is promoted with the increase of the surface area of the catalyst if the same amount of the catalyst metals is supported thereon.
In preparing the above catalysts, the metal elements are supported onto the inorganic support by reducing the ions containing the catalyst metals to be supported, to the metal elements by means of a reducing agent.
Since however, such a reducing agent as lithium aluminum hydride and sodium borohydride ordinarily employed is too strong, the particle size of the metals produced by the reduction increases and the particle size distribution becomes broader.
In other words, the conventional method has the drawbacks such that the number of the particles decreases to lower the surface area per unit weight of the metal so that the catalytic activity is also made to be lowered, and the particle size becomes considerably uneven.
Various kinds of alloy catalysts having high catalytic activity have been heretofore proposed after the investigation of the combination of supported metals (for example, Japanese patent application No. 59-141169).
However, even in these catalysts, the catalyst metals thereof are requested to have a small particle size, that is, a large specific surface area for elevating the activity.
Moreover, the alloy catalysts are generally prepared by alloying an alloy component with a noble metal already supported on the support. It is important from this viewpoint to prepare the catalyst supported with a high surface area noble metal having narrow particle size distribution, that is, a uniform particle size.
Having the uniform particle size is important to obtain an alloy catalyst having particles of uniform alloy composition.
These catalysts have the drawback that they are likely to be exposed to a high temperature so that the activity may be lost with the lapse of time to shorten the catalyst life if they do not have the resistance to a sintering reaction.
In order to overcome these drawbacks, such a sulfur-containing and relatively weak reducing agent as a thiosulfate salt and a metabisulfite has been proposed (U.S. Pat. No. 4,956,331 and European patent publication No. 0329628). Although these sulfur-containing reducing agent exhibit more excellent catalytic activity and resistance, another reducing agent having much more excellent activity and resistance is of course desirable.